<i>Acinetobacter baumannii</i> α2–6‐Legionaminyltransferase‐Catalyzed Synthesis Reveals Legionaminic Acid Form Selectivity by Antibodies in Pooled Human IgGs

Legionaminic acids (Legs) are prokaryote-specific nine-carbon acidic monosaccharides belonging to the nonulosonic acid (NulO) family. Various Leg forms are found in the polysaccharides or glycoproteins produced by numerous pathogenic bacteria, including capsular polysaccharides of carbapenem-resistant Acinetobacter baumannii. Hypothetic legionaminyltransferases have been predicted from gene sequences but none has been biochemically validated. In this study we report the first biochemical characterization of a legionaminyltransferase. We demonstrate that AbGtr18 from an A. baumannii K8 strain is a donor promiscuous α2-6-legionaminyltransferase selective for α-N-acetylgalactosamine-glycoside (GalNAcαOR) acceptors. In addition, streamlined chemical synthetic routes, including regio-selective triflylation and azide substitution processes, were developed to prepare precursors or chemoenzymatic synthons of various forms of Legs. Applying the novel α2-6-legionaminyltransferase AbGtr18 in one-pot multienzyme (OPME) synthesis with or without additional chemical derivatization enabled rapid construction of a focused library of NulO-glycosides (NulOα2-6GalNAcαOR) containing nine NulO forms. Binding and inhibition studies with these compounds revealed selective Leg forms recognized by the antibodies in the pooled human immunoglobulin (IgGs), reflecting natural occurrence of these Leg forms and indicating their prior encounters by human immune systems. AbGtr18 thus emerges as both a potential target for antibiotic development and a powerful biocatalyst for accessing synthetically challenging NulO-glycosides with therapeutical potentials.